Which language/library(s) support date/time/localization well?

If you've tried to look at the edge cases of timezones, you've probably noticed it's kind of a pain with daylight saving time, time zone changes, etc.

But there's also things like adding a month to a date (how many days?) or subtracting dates across a DST switch.

Which programming languages or libraries do you feel handled it well? What do you like about it?